FIG. 1A is a simplified block diagram showing an ER transmitter (TX) 1 architecture that includes an amplitude modulation (AM) chain and a phase modulation (PM) chain. Bits to be transmitted are input to a bits to polar converter 2 that outputs an amplitude signal, via propagation delay (PD) 3, to an amplitude modulator (AM) 4. The AM 4 (after digital to analog conversion) supplies a signal for controlling the output level of a TX power amplifier (PA) 6 through the use of a controllable power supply 5. The bits to polar converter 2 also outputs a phase signal via propagation delay 3 to a frequency modulator (FM) 7, which in turn outputs a signal via a phase locked loop (PLL) 8 to the input of the PA 6. The transmitted signal at an antenna 9 is thus generated by simultaneously using both phase and amplitude components. The benefits that can be gained by using the ER transmitter architecture include a smaller size and an improved efficiency.
As can be appreciated, the supply voltage of the PA 6 should be amplitude modulated with high efficiency and with a wide bandwidth.
Discussing the power supply 5 and PA 6 now in further detail, high efficiency TX architectures, such as the polar loop modulation TX, typically rely on highly-efficient but non-linear power amplifiers, such as switch mode power amplifiers (SMPA), for example a Class E SMPA, or they rely on normally linear power amplifiers that are driven into saturation, such as the saturated Class B power amplifier. In these architectures the amplitude information is provided by modulating the supply voltage of the PA 6 by means of a power regulator that is connected between a DC supply or power source, typically a battery, and the PA 6, as shown in greater detail in FIG. 1B.
In FIG. 1B the output of the power supply 5, Vpa, should be capable of tracking a rapidly varying reference voltage Vm. As such, the power supply 5 must meet certain bandwidth specifications. The required bandwidth depends on the system in which the transmitter 1 is used. For example, the required bandwidth exceeds 1 MHz (dynamic range ˜17 dB for a given power level) for the EDGE system (8PSK modulation), and exceeds 15 MHz (dynamic range ˜47 dB for a given power level) for the WCDMA (wideband code division multiple access) system. As may be appreciated, these are very challenging requirements. A typical waveform (RF envelope in the EDGE system) that must be tracked is shown in FIG. 2, where the modulating voltage (Vm) is shown as varying between minimum and peak values (the typical rms and average values are also shown).
It is noted that in the GSM system the modulation is GMSK, which has a constant RF envelope, and thus for a given power level imposes no particular constraints in terms of bandwidth on the power supply 5.
In general, there are two primary techniques to implement the power supply 5. A first technique, shown in FIG. 3, uses a linear regulator implemented with a summing junction 10, a driver 12 and a power device 14. While a high bandwidth can be obtained, the efficiency is low due to the voltage drop (Vdrop) across the power device 14.
A second technique, shown in FIG. 4, would be to use a switch mode regulator. In this technique, which is not admitted has been previously used in a polar or ER transmitter, a step-down switching regulator 16 would include a Buck-type or similar converter 18 and voltage-mode control circuitry 20. The PA 6 is shown represented by its equivalent resistance Rpa. While the efficiency of the switch mode regulator 16 can be very high, the required bandwidth would be difficult or impossible to obtain. More specifically, if one where to attempt the use of the switching regulator 16 it would require a very high switching frequency (e.g., at least approximately five times the required bandwidth, or 5-10 MHz or more for EDGE and over 80 MHz for WCDMA). While a switching frequency of 5-10 MHz would be very technically challenging (typical commercial DC-DC converters operate with maximum switching frequencies in the range of about 1-2 MHz), a DC-DC converter having a 100 MHz switching frequency, for example, is currently impractical to implement, especially in low cost, mass produced devices such as cellular telephones and personal communications terminals.
In U.S. Pat. No. 6,377,784 B2, “High-Efficiency Modulation RF Amplifier”, by Earl McCune (Tropian, Inc.), there is purportedly described high-efficiency power control of a high-efficiency (e.g., hard-limiting or switch-mode) power amplifier in such a manner as to achieve a desired modulation. In one embodiment, the spread between a maximum frequency of the desired modulation and the operating frequency of a switch-mode DC-DC converter is purportedly reduced by following the switch-mode converter with an active linear regulator. The linear regulator is said to be designed so as to control the operating voltage of the power amplifier with sufficient bandwidth to faithfully reproduce the desired amplitude modulation waveform. The linear regulator is said to be further designed to reject variations on its input voltage even while the output voltage is changed in response to an applied control signal. The rejection is said to occur even though the variations on the input voltage are of commensurate, or even lower, frequency than that of the controlled output variation. Amplitude modulation is said may be achieved by directly or effectively varying the operating voltage on the power amplifier while simultaneously achieving high efficiency in the conversion of primary DC power to the amplitude modulated output signal. High efficiency is purportedly enhanced by allowing the switch-mode DC-to-DC converter to also vary its output voltage such that the voltage drop across the linear regulator is kept at a low and relatively constant level. It is said that time-division multiple access (TDMA) bursting capability may be combined with efficient amplitude modulation, with control of these functions being combined, and that the variation of average output power level in accordance with commands from a communications system may also be combined within the same structure.